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Russian Chemical Bulletin, International Edition, Vol. 65, No. 11, pp. 2682—2685, November, 2016

Generation of oxodiazonium ions 6.* Unexpected formation of tetrazole 1oxides** M. S. Klenov,a A. M. Churakov,a I. V. Fedyanin,b and V. A. Tartakovskya aN.

D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian Federation. Fax: +7 (499) 135 5328. Email: [email protected] bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 ul. Vavilova, 119991 Moscow, Russian Federation. Fax: +7 (499) 135 6549. Email: [email protected] (E)2[2,2Bis(tertbutylNNOazoxy)1(methylsulfinyl)vinyl]1isopropoxydiazene 1oxide reacts with boron trifluoride etherate (BF3•Et2O) producing 2tertbutylNnitro2H tetrazolo5carboxamide 4oxide and 2tertbutyl2Htetrazolo5carbonitrile 4oxide but not the expected 1,2,3,4tetrazine 1,3dioxide derivative. This reaction direction can be explained by cationic domino cyclization, the key stage of which is coupling of the oxodiazonium ion with the geminal MeS(O) group. Structure of Nnitrocarboxamide was confirmed by Xray diffrac tion analysis. Key words: azoxy compounds, sulfoxides, tetrazole 1oxides, oxodiazonium ion, 1,2,3,4 tetrazine 1,3dioxides, domino cyclization.

Our research group performs systematic studies on the synthesis and properties of 1,2,3,4tetrazine 1,3dioxides (TDO).2—5 Compounds of this type attract interest as physiologically active substances6,7 and energetic materi als.8—10 [1,2,3,4]Tetrazino[5,6e][1,2,3,4]tetrazine 1,3,6,8 tetraoxide (1) is of special demand as highenergy com pound (Scheme 1). We implemented first quantumchem ical calculations of compound 1 in 1999.11 Recent com putational studies12—15 predict for this compound high energy release characteristics. The retrosynthetic analysis (Scheme 1) shows that TDO 2 bearing in the neighboring positions the amino and tertbutylNNOazoxy groups can serve as a suitable precursor for compound 1. In turn, compound 2 can be synthesized from TDO 3a,b bearing the leaving group X.

Recently, we elaborated synthetic methods to access (E)2[2,2bis(tertbutylNNOazoxy)1(methylthio) vinyl]1isopropoxydiazene 1oxide (4a)3 and (E)2[2,2 bis(tertbutylNNOazoxy)1(methylsulfinyl)vinyl]1 isopropoxydiazene 1oxide (4b).16 Compounds 4a and 4b can be regarded as potential precursors for compounds 3a and 3b, respectively. Reaction of alkene 4a with boron trifluoride etherate (BF3•Et2O) expectedly results in TDO 3a (Scheme 2).3 We suggest that the reaction proceeds via intermediate oxodiazonium ion A, which undergoes ring closure to the TDO cycle. We earlier detected formation of intermediate oxodiazonium ion in the similar reactions with the aromatic substrates.17 At the same time, reaction of 4b with BF3•Et2O failed to give target TDO 3b (Scheme 3).

Scheme 1

X = SMe (3a), S(O)Me (3b)

* For Part 5, see Ref. 1. ** Dedicated to Academician of the Russian Academy of Sciences O. M. Nefedov on the occasion of his 85th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2682—2685, November, 2016. 10665285/16/65112682 © 2016 Springer Science+Business Media, Inc.

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Scheme 2

Scheme 3

Scheme 5

Since the knowledge on reactivity of oxodiazonium ion is very important for planning syntheses of new hetero cyclic systems, in the present work we performed a de tailed study of this reaction. Results and Discussions The major reaction products formed in the reaction of 4b with BF3•Et2O are tetrazoles 5 (34%) and 6 (20%) (Scheme 4). Some amount of resinous products is also formed. Scheme 4

i. BF3•Et2O, 25 °C, 7 days.

The absence of compound 3b among the reaction prod ucts can be rationalized as follows. The intermediate oxo diazonium ion B formed upon the reaction between alk ene 4b and BF3•Et2O reacts with geminal methylsulfinyl group faster than with tertbutylNNOazoxy group (Scheme 5). Subsequent attack of the azoxy group oxygen at the sulfur atom of cyclic intermediate C results in bi cyclic cation D, which undergoes ring opening to cation E. For consideration of further transformations of cation E, it is convenient to formally disconnect its C—C bond into fragments F and G. Plausible transformations of the F fragment are shown on Scheme 6. This cation can react in two directions both

of which give rise to the same product I. The first direction involves ring closure of cation F into cyclic cation H further undergoing the tertbutyl cation elimination to produce tetrazole Noxide moiety I. An alternative pathway is the sequence of the following transformations: abstraction of the tertbutyl cation to give diazo compound J, cycliza tion of the latter into tetrazole Noxide K, and final isomer ization of K into more thermodynamically favorable tetra zole Noxide I. It should be emphasized that we have previously observed an intramolecular ring closure of the diazo and tertbutylNNOazoxy groups into tetrazole Noxide cycle and 1,2shift of the tertbutyl group in this cycle.18 Decomposition of oxothiadiazole cycle G proceeds ap parently by two directions (Scheme 7). In the first case, decomposition of G leads to nitrile L and sulfinyl nitrite M. In the second case, cycle G undergoes a ring opening to nitrimine N, which hydrolyzes with simultaneous sub stitution of the sulfinyl group producing nitramide О. Tetrazole 5 is a light yellow crystalline substance melt ing at 144—146 °C with decomposition. Structure of com pound 5 was established by IR and NMR spectroscopy, high resolution mass spectrometry and confirmed by Xray diffraction analysis (Fig. 1, Table 1).

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Table 1. Selected bong lengths (d) for tetrazole 5

Scheme 6

Bond O(1)—N(4) O(2)—C(6) O(3)—N(6) O(4)—N(6) N(1)—N(2) N(1)—C(5) N(2)—N(3) N(3)—N(4) N(4)—C(5) N(5)—N(6) N(5)—C(6) C(5)—C(6)

d/Å 1.2805(19) 1.2020(2) 1.2130(2) 1.2130(2) 1.3160(2) 1.3230(2) 1.3290(2) 1.3310(2) 1.3670(2) 1.3800(2) 1.3870(2) 1.4850(2)

According to Xray diffraction analysis, the bond lengths and bond angles of tetrazole Noxide moiety of compound 5 are practically the same as those found earli er for Smethyl 2tertbutyl2Htetrazole5carbothioate 4oxide.3 The nitramide hydrogen atom participates in strong hydrogen intramolecular bonding (the N(5)...O(1) distance is 2.686(2) Å). 1H NMR spectrum (acetoned ) of tetrazole 5 exhib 6 its a singlet signal of the tertbutyl protons at δ 1.85. A nitrogen atom of the nitro group resonates in 14N NMR spectrum (acetoned6) at δ –38. IR spectrum of com pound 5 shows absorptions attributed to symmetrical and asymmetrical vibrations of the nitro (KBr, ν 1307 and 1605 cm–1) and carboxyl (KBr, ν 1742 cm–1) groups. Physicochemical parameters of tetrazole 6 are identi cal (m.p., Rf, 1H NMR spectral data, mass spectral data) to those of a reference sample obtained following the known procedure.18 In summary, we found that the reaction of alkene 4b with BF3•Et2O gives 2tertbutylNnitro2Htetrazolo 5carboxamide 4oxide (5) and 2tertbutyl2Htetrazolo 5carbonitrile 4oxide (6) but not the expected product, the 1,2,3,4tetrazine 1,3dioxide derivative. A mechanistic rationale for this unexpected transformation is a cationic domino cyclization involving oxodiazonium ion.

Scheme 7

O(2) O(3) С(3) С(4)

N(1)

Experimental

С(6) N(6)

С(5)

N(2) N(5) С(1) H(5)

С(2) N(3)

O(4)

N(4) O(1)

Fig. 1. Xray crystal structure of tetrazole 5. Thermal ellipsoids for nonhydrogen atoms are drawn at 50% probability level. Se lected bong lengths are given in Table 1.

1H and 14N NMR spectra were recorded with a Bruker AM300 instrument at working frequencies of 300.13 and 21.69 MHz, respectively. The chemical shifts are given in the δ scale relative to SiMe4 (1H) and MeNO2 (14N, internal standard, downfield shifts are negative). IR spectrum was recorded on a Specord M802 spectrophotometer. High resolution mass spectrometry was performed with a Bruker micrOTOF II instrument. The course of the reaction was monitored by TLC on precoated Silu fol UV254 plates. Preparative TLC was performed with Silpearl UV254 silica gel. (E)2[2,2Bis(tertbutylNNOazoxy)1

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(methylsulfinyl)vinyl]1isopropoxydiazene 1oxide (4b) was synthesized by the known procedure.16 Xray diffraction analysis of tetrazole 5 was performed with a Bruker Apex DUO automated diffractometer (λ(MoKα) = = 0.71073 Å, ω scan mode). Colorless crystals C6H10N6O4 (M = 230.2) at 120 K are monoclinic, P21/c space group, a = 5.6254(11) Å, b = 8.1196(16) Å, c = 21.338(4) Å, β = 93.44(3), V = 972.9(3) Å3, Z = 4 (Z´ = 1), dcalc = 1.572 g cm–3. An empirical absorption corrections based on semiequivalent re flections was applied by SADABS program. Further calculations were based on 2123 independent reflections (Rint = 0.0358) from 12412 measured reflections. The structure was solved by direct method and refined by full matrix least squares against F 2hkl using anisotropic thermal parameters for all nonhydrogen atoms. A position of hydrogen atom bonded to the nitrogen atom was revealed from the difference Fourier synthesis and refined iso tropically. The tertbutyl group hydrogen atoms were placed in geometrically calculated positions and refined in a riding model. The final divergence parameters are R1 = 0.0485 (for 1948 re flections with I > 2σ(I) and 2θ < 54°), wR2 = 0.1198, and GOOF = 1.010. The structure was solved and refined with SHELX program package, version SHELXL2014/7.19 Crystal lographic data for structure 5 were deposited with the Cam bridge Crystallographic Data Center (CCDC 1480096). Caution! Compounds 5 and 6 are potential explosives and have to be handled with care. Reaction of sulfoxide 4b with BF3•Et2O. Freshly distilled BF3•Et2O (3 mL) was added by one portion to a solution of sulfoxide 4b (325 mg, 0.83 mmol) in anhydrous CH2Cl2 (1 mL) at 25 °C under dry argon. The reaction mixture was kept at 25 °C for 7 days and concentrated in vacuo (1 Torr). The residue was coevaporated with anhydrous CH2Cl2 (2 mL). Purification of the residue by preparative TLC (elution with petroleum ether— AcOEt (2 : 1, then 1 : 1)) afforded 64 mg (34%) of tetrazole 5 and 27 mg (20%) of tetrazole 6. 2tertButylNnitro2Htetrazole5carboxamide 4oxide (5). Light yellow crystals, m.p. 144—146 °C (decomp.) (from acetone). MS (ESI), m/z: 253.0657 [M + Na]+; calculated for C6H10N6O4, [M + Na]+: m/z: 253.0656. IR (KBr), ν/cm–1: 1307 and 1605 (NO2); 1742 (C=O); 3478 (NH). 1H NMR (ace toned6), δ: 1.85 (s, 9 H, CMe3). 14N NMR (acetoned6), δ: –38 (NO2, Δν1/2 = 130 Hz). 2tertButyl5cyanotetrazole 4oxide (6). Colorless crystals, m.p. 102—105 °C (from petroleum ether—CH2Cl2). Physico chemical parameters (m.p., Rf, 1H NMR spectroscopy and mass spectrometry data) are identical to a reference sample obtained by the published procedure.18

This work was financially supported by the Russian Science Foundation (Project No. 145000126).* References 1. V. P. Zelenov, A. A. Voronin, A. M. Churakov, M. S. Klenov, Yu. A. Strelenko, V. A. Tartakovsky, Russ. Chem. Bull. (Int. Ed.), 2012, 61, 351 [Izv. Akad. Nauk, Ser. Khim., 2012, 349]. 2. A. M. Churakov, V. A. Tartakovsky, Chem. Rev., 2004, 104, 2601. * For other publications on this topic see Refs 20—24.

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Received May 23, 2016 in revised form July 1, 2016